1. Field of the Invention
The invention concerns integral-type threaded connections of two metal tubes.
2. Discussion of the Background
Integral threaded connections involve a male element, provided at the end of a first tube, connected to a female element provided at the end of a second tube without any intermediate sleeve-type part.
Such integral threaded connections are known, especially for use in assembling columns of production or liner tubes or of drill-rod strings in oil or gas wells or in similar wells, such as wells for geothermal power.
U.S. Pat. No. 4,521,042 describes an integral threaded connection provided with:
cylindrical male and female threads with two radially distinct stages;
three pairs of transversal annular surfaces, one being a middle pair between the two thread stages, one an outside pair toward the female free end and one an inside pair toward the male free end;
and three pairs of conical sealing surfaces, one toward the male free end, one toward the female free end and one comprised by the middle pair of transversal surfaces.
Each pair of transversal surfaces or of sealing surfaces is comprised of two mating surfaces, one being on the male element and the other on the female element.
The transversal surfaces of the middle pair of transversal surfaces are braced against the connection and have the function of absorbing the tightening torque, of imposing tension on the threads, of ensuring central sealing and of defining the final connection position for the sealing surfaces.
The inside and outside pairs of transversal surfaces provide reinforcement for the middle pair in the event of overtightening or of overload during service.
In the remainder of the present description, bearing surfaces are defined as such transversal surfaces which are in braced relationship or which have the potential to be in braced relationship. These bearing surfaces have substantially transversal orientation relative to the axis of the connection, and they can be ends of tubes or can originate from shoulders on the inside surface of the female element or on the outside surface of the male element.
The terms xe2x80x9ctransversalxe2x80x9d and xe2x80x9clongitudinalxe2x80x9d relate in the rest of the present description to the direction of the axis of the connection.
The middle pair of bearing surfaces is thus comprised of very open conical surfaces, which are convex on one element and concave on the other, such that they generate radial stresses while they are being brought into braced relationship.
The sealing surfaces are organized into pairs of male and female surfaces, which interfere radially, one against the other, with high metal-to-metal contact pressure. These pairs are designed to ensure sealing of the column with respect to the internal fluid and/or with respect to the external medium, even when the fluid pressures are high.
The diametrical interference between paired reference points of two rotational surfaces of a connection is generally defined as the difference in diameter between these points measured when the elements are not joined. The diametrical interference is counted positively when the surfaces, once joined, interfere radially with each other and develop contact pressure, the contact pressure being roughly proportional to the diametrical interference.
A connection such as the one described in patent U.S. Pat. No. 4,521,042 is costly to produce for various reasons:
The production of the male and female elements, generally by machining, is necessarily a lengthy process given the complex geometry to be produced and the precision necessary for the connection to function properly.
For example, the production of overhanging shoulders with concave bearing surfaces and acute angles at the foot of the shoulders is delicate and may require the use of specific machining stages and/or special tools.
Furthermore, synchronization of the action of multiple surfaces and stop devices to achieve proper, reproducible functioning of the connection is not obvious.
Improper functioning of the connection can be due to poorly chosen dimension figures particularly considering the plastic deformations of the male and female elements when they are joined.
Non-reproducible functioning of the connection can be due to poorly chosen dimensional figures particularly considering the manufacturing tolerances.
Finally, in order to house the two stages of threading and the various bearing surfaces, production of the male and female elements generally requires tubes whose ends have been upset, for example by forging, compared to the bodies of the tubes, which is a relatively costly operation.
Another integral-type threaded connection called xe2x80x9cVAM(copyright) ACE XSxe2x80x9d is described on pages 28 to 31 of the VAM Catalogue No. 940 published in July 1994 by Vallourec Oil and Gas.
On the male element, moving toward the end of the first tube, the connection of two tubes described in this document has an outer shoulder with a flat, annular bearing surface perpendicular to the connection axis, tapered male threading with trapezoidal threads tapered {fraction (1/16)} (6.25%) on the diameter, a cylindrical connecting lip beyond the threading and a tapered male sealing surface tapered 10% on the diameter. The end surface of the male element, which is also the end surface of the tube, is a slightly concave tapered surface, the points of this surface in the area of the sealing surface being slightly prominent when compared to those located in the area of the inner surface of the tube.
On the female element, moving toward the end of the second tube, the connection has an inner shoulder with a slightly tapered convex bearing surface whose amount of taper corresponds to that of the end surface of the male element, a tapered female sealing surface whose amount of taper corresponds to that of the male element and tapered female threading with trapezoidal threads that is complementary to the threading of the male element. Beyond the threading, the female element ends in an end surface perpendicular to the connection axis.
When these two tubes are joined, the external threading of the male element is screwed into the internal threading of the female element, the end surface of the male element forms an inner pair of abutted bearing surfaces with the bearing surface of the female element, the end surface of the female element forms an outer pair of abutted or roughly abutted bearing surfaces with the bearing surface of the male element, and the male sealing surface interferes radially with the female sealing surface.
Owing to its two pairs of bearing surfaces, this type of connection makes it possible to impart a very high make-up torque and to obtain good compression and bending strength. Nevertheless, this type of connection remains rather costly to produce if one wants it to function properly and in reproducible fashion.
FR 2 364 322 describes yet another integral threaded connection whose general arrangement is quite similar to the xe2x80x9cVAM(copyright) ACE XSxe2x80x9d connection having a shoulder on each element with an annular bearing surface, tapered threading, a sealing surface, and an annular end surface wherein the male sealing surface is located between the male threading and annular end surface while the female sealing surface is located between the female annular bearing surface and threading.
The connection described in patent FR 2 364 322 differs from the xe2x80x9cVAM(copyright) ACE XSxe2x80x9d by the presence on each element of a second sealing surface at the end of the element and by the fact that the pair of tapered bearing surfaces can be either the inner pair or the outer pair depending on the anticipated service conditions.
According to the document FR 2 364 322, the sealing surface on the side of the end is rounded and the sealing surface on the side of the shoulder is tapered; the rounded sealing surface of one element cooperates with the tapered sealing surface of the other element to form a pair of sealing surfaces.
The surfaces of the pair of tapered bearing surfaces are abutted first during screwing to take advantage of the relative flexibility of this type of stop device and to amplify the contact pressure of the pair of sealing surfaces closest to the pair of tapered bearing surfaces.
The pair of flat bearing surfaces serves as a reinforcement and constitutes a very stiff safety stop, all the more so because their surface area is greater than the surface area of the pair of tapered bearing surfaces.
So that the pair of tapered bearing surfaces abuts first during make-up, the patent provides that the distance between the end surface and the bearing surface of the element whose end surface is tapered is greater by 0.05% to 0.25% than the corresponding distance on the other element.
This type of connection is also costly to produce.
With the present invention, we sought to produce an integral threaded connection with tapered threads, two pairs of bearing surfaces, and at least one pair of sealing surfaces whose geometry is optimized but that is economical to produce.
More particularly, we sought to induce maximum contact pressure at the sealing surfaces.
We also sought to achieve good operational synchronization of the two pairs of bearing surfaces by privileging the abutment during connection of one pair of bearing surfaces, always the same.
We also sought to use sealing surfaces that are less susceptible to seizing.
All these properties are basically obtained by combining the features of claim 1 of this invention.
The integral threaded connection of two metal tubes in accordance with the invention comprises a male element at the end of a first tube and a female element at the end of a second tube.
The male element comprises, moving toward the free end of the first tube:
an outer shoulder with an annular bearing surface called xe2x80x9cmale outer,xe2x80x9d
an external tapered threading called xe2x80x9cmale threading,xe2x80x9d
an outer sealing surface called xe2x80x9cmale sealing surface,xe2x80x9d
and an annular male end surface that is also the end surface of the first tube.
The female element comprises, moving toward the free end of the second tube:
an inner shoulder with an annular bearing surface called xe2x80x9cfemale inner,xe2x80x9d
an inner sealing surface called xe2x80x9cfemale sealing surfacexe2x80x9d adapted to the male sealing surface,
internal tapered threading called xe2x80x9cfemale threadingxe2x80x9d complementary to the male threading,
and an annular female end surface that is also the end surface of the second tube.
The male threading is screwed into the female threading.
The male sealing surface interferes radially with the female sealing surface.
The male end surface forms an inner pair of bearing surfaces with the female inner bearing surface and the female end surface forms an outer pair of bearing surfaces with the male outer bearing surface.
The end and bearing surfaces of each male and female element are flat surfaces positioned perpendicular to the connection axis.
The male sealing surface is separated from the male threading by a lip that has an external tapered surface whose magnitude of taper is equal to that of the male threading and whose generatrix is roughly in line with the tangent to the roots of the thread of the male threading while remaining on the side of the connection axis with respect to a line tangential to said male thread roots.
The surface of the male sealing surface is interior or tangent to the surface extending the tapered lip surface.
The distance between the male end surface and the male outer bearing surface is adapted to the distance between the female end surface and the female inner bearing surface so that during screwing the inner pair of bearing surfaces are abutted first.
This type of configuration makes it possible to protect the male sealing surface from blows likely to cause the connection to leak, to increase the functional features of the connection, to make this operation reproducible and reliable, and to simplify production of the elements all at the same time.
Preferably, the distance between the generatrix of the tapered lip surface and the tangent to the roots of the male threads is less than or equal to 0.20 mm.
Also preferably, the amount of taper of the male and female threads compared to the diameter is between 6.25% and 20%.
Preferably again, the male sealing surface is a tapered surface coaxial to the tapered lip surface and with a greater amount of taper than this lip surface and the female sealing surface is also a tapered surface with an amount of taper roughly identical to that of the male surface.
Very preferably, the amount of taper of the male sealing surface compared to the diameter is between 25% and 75%.
Preferably, the male and female threads are each formed by a single threaded portion.
In a more costly variation, they can each be formed by two-stepped threaded portions, the tapered lip surface then being roughly in line with the threaded portion of the male threading to which it is adjacent.
Preferably, the surfaces of the outer pair of bearing surfaces are abutted against each other on the connection like the surfaces of the inner pair of bearing surfaces. The first pair of bearing surfaces to be abutted, i.e., the inner pair of bearing surfaces, absorbs the major portion of the effective make-up torque; the outer pair of abutted bearing surfaces absorbs the remainder of the make-up torque.
Alternatively, the surfaces of the outer pair of bearing surfaces are almost in contact with each other on the connection.
This means that the surfaces of the outer pair of bearing surfaces are less than one-tenth of a mm apart.
Whether the surfaces of both pairs of bearing surfaces are abutted on the connection or only those of the inner pair of bearing surfaces, the outer pair of bearing surfaces abutted or almost in contact is there as an immediate reinforcement in order to absorb the residual screwing stresses or the supplementary service stresses without risking plasticizing the metal of the elements.
Advantageously, in order to ensure that the inner pair of bearing surfaces abuts first during screwing, prior to joining, the distance between the male end surface and the male outer bearing surface is slightly longer by a determined deviation xcex94L than the distance between the female end surface and the inner female bearing surface.
Preferably, the deviation xcex94L is a decreasing linear function of the outer diameter of the tubes of the connection and an increasing linear function of their thickness.
The invention also concerns a machining production process for the integral threaded connection targeted by the invention in which both the end and bearing surfaces of a given element are machined during the same machining stage.
This type of process makes it possible to obtain good precision with respect to the distance between these two surfaces.
Advantageously, in the case of machining the male element, the tapered lip surface and the tapered surface that encases the thread crests of the male threading can be machined during the same machining stage.